The water in steam boiler systems is lost through air vents, steam leaks, the condensation of steam that is purposely not returned to avoid contamination of the boiler and through consumption in industrial process using live steam which cannot be returned to the boiler.
The quantity of water to be removed from the steaming boiler, for the necessary purpose of reducing the amount of solid matter present in the boiler, varies greatly with different steam boiler plants. However, the losses in water must be replaced with make-up water if the boiler is to continue operating. The greater the load on the boiler the sooner a high concentration of solid matter occurs in the boiler water.
It is known in this art that 100% of the dilution required can be obtained by withdrawing boiler water only from the bottom of the boiler. It is, however, not possible to accomplish 100% dilution by using so-called "surface blow-off" method. The American Boiler and Affiliated Industries have established concentration limits beneath which satisfactory steam production can be expected.
In general, a maximum total solids (which includes dissolved and suspended solids) content of about 3500 PPM, maximum suspended solids content of about 300 PPM and a maximum alkalinity of 700 PPM for boilers operating at or below 300 psig pressure are acceptable.
Expensive and complicated automatic blowdown systems are known in the prior art, but these systems are not entirely reliable, calling for operations which may create risk of live steam being released to the atmosphere or conditions which otherwise do not comply with various codes and which are therefore not approved by building authorities. In the U.S. Pat. No. 3,908,605 by C. M. Andersen, there is described an apparatus and method for automatically blowing down a boiler. The boiler water containing sediments is withdrawn from the bottom of the boiler into an intermediate or holding tank. This system is severely limited to hourly blow-down capacity due to ineffeciency in heat exchange mode. This fault makes Andersen's unit completely impractical for application to high pressure boilers using large quantities of make-up water.
The sequence of steps are: pressurizing the holding tank with steam from the boiler, isolating the tank to condense the steam, blow down the boiler into the tank, expelling blowdown water and sediment from the tank and again repressurizing the tank to repeat the cycle.
This system also requires the use of considerable quantities of steam to bring the holding tank to the desired pressure, the heat of which is lost in the isolation and condensing steps and also to force the blowdown water from the bottom thereof.
Sufficient steam under pressure is used in receiving tank to create a partial vacuum therein for withdrawal of the next cycle of blowdown water, considerable heat is thereby wasted. Furthermore, the time cycle requires the use of large quantities of the cooling water for the holding tank and it is impossible to adapt such a system to boilers which require thousands of pounds of water blowdown every hour.
According to this invention, a receiving tank is used for the purpose of withdrawing the blowdown water from the boiler without significant cooling. Rather, the blowdown water is immediately cycled from the full receiving tank to a heat exchanger at a specific flow rate and the heat exchanger is isolated from the boiler so that the rate of cooling of the water is under direct and independent control. The instant invention is adapted for use with high capacity multiple-boiler installations wherein both surface water blow-off and bottom blowdown from each boiler are necessary.